1. Field of the Invention
This invention generally relates to methods of driving LED and OLED devices, and more particularly, to some unique concepts to drive LED and OLED devices with low cost circuits while providing high efficiency power conversion and comprehensive dimming control performance.
2. Description of the Related Art
Light Emitting Diode (LED hereafter) and Organic Light Emitting Diode (OLED hereafter) are bringing revolutionary changes to the lighting industry and the whole world. High efficiency, compact size, long lifetime and minimal pollution etc. are some of the main advantages that provide people elegant lighting solutions and in the meanwhile perfectly into the green power initiative. Because LED and OLED are all made with solid substances, they are also called Solid State Lighting (SSL hereafter) devices. The inherent mechanical robustness of SSL devices together with the features described above also enable themselves to provide more reliable solutions that other lighting devices cannot do, and create many new applications in our daily life.
Despite the technical advantages of the LED and OLED, high cost of the devices and especially the total lighting system solutions is the most critical factor that hinders the fast growth of the SSL applications. Apart from the device itself, the drive circuitry that converts the input electrical power from a commonly available format to a format that provides suitable voltage and current to the device, consists a large part of the system cost. In applications that the input power is from the mains AC power line of 110V or 220V, the cost of the drive circuitry would be more significant because of the complexity of the power conversion process that very often includes Power Factor Correction (PFC hereafter) circuit, DC to DC conversion, and dimming control circuit in particular.
FIG. 1 shows a typical approach of an AC powered LED drive system. For simplicity of the description, the figure shows only the power circuit architecture. As shown in the figure, inductor 160, power MOSFET switch 170, diode 180, and capacitor 120 comprise a boost type PFC circuit that converts the voltage rectified by bridge rectifier 110 from the AC line input VAC, to a DC voltage VDC while maintaining the input current from the AC line in a sinusoidal wave shape and in phase with the AC input voltage. As well known by the skilled in the art, PFC function is mandatory by European standard for all the electric apparatuses that draws 75 W or above from the mains AC line, and very soon such requirement will be extended to lower power level. The output voltage of the PFC stage is normally around 180 VDC for 110V AC input, and 380V for 220V AC input. These voltage levels are defined such that they are slightly higher than the maximum AC input peak which is VACNOM·110%·√2, in order to maintain proper operation of the PFC circuit. Lower than this level will result in the possibility of uncontrolled conduction of the diode 180. Here VACNOM represents the nominal mains AC voltage, i.e. 110V or 220V (240V for British system).
Since the operating voltage of LED device or most LED strings is lower than the PFC output voltage, a DC to DC conversion stage is employed to convert the PFC output voltage VDC to a lower DC voltage that suitable for driving the LED devices. MOSFET switch 130, power transformer 50, rectifier diode 220 and capacitor 230 in FIG. 1 forms a fly back type of DC to DC conversion stage. The voltage established on capacitor 230 is the converted voltage for LED drive. Apart from the illustrated fly back converter configuration, other types circuit topology such as forward, push-pull, half bridge, or full bridge can also be employed to perform the DC to DC conversion function. The operating principles of those circuits are well known to the skilled in the art and will not be elaborated herein.
In lighting applications LED or OLDE are normally current controlled devices of which the light output of the device is proportional to the forward current flowing through it. On the other hand in the forward conduction region of the device the dynamic impedance is very low, i.e. a relatively small change of the forward voltage will result in a large change of the forward current. In order to maintain the forward current of the device at a desired value or control the current at different level according dimming requirement, a drive circuit is normally employed to control the current flowing to the LED device as shown in FIG. 1. Note that the LED symbol in the figure represents an LED lighting assembly in general. It could be a single LED or OLED device, or an LED string or OLED string consisting multiple devices connected in series.
It is obvious that such approach involves multiple power conversion stages and utilizes multiple power devices to accomplish the whole power control process. The system efficiency suffers from the multiple stage power conversion, and the cost of the system is too high compared with other lighting solutions to prevent its wide adoption in many applications, especially the high volume general lighting area. Therefore it is the intention of this invention to introduce an innovative LED drive concept with high operating efficiency and lower system cost to better fit the market needs.